1. Field of the Invention
The present invention relates to an inorganic dispersant, a stabilizer for suspension polymerization comprising said, dispersant, polymer particles, unsaturated polyester resin compositions and toner compositions, using said polymer particles, more specifically, to an inorganic dispersant having a high specific surface area and a high surface activity, a stabilizer for suspension polymerization comprising said dispersant, polymer particles having a uniform and sharp particle size distribution obtained by the use of said dispersant, unsaturated polyester resin compositions and toner compositions, using said polymer particles.
2. Description of the Prior Art
For the production of spherical polymer particles, for example, a method for suspension polymerization and a method for dissolving a polymer in a solvent, dispersing the resulting solution in water to form a suspension and removing the solvent from the suspension have been known.
On the other hand, as an inorganic dispersant, effects of water-difficult soluble phosphates for suspension polymerization have been known from of old. Especially, a suspension polymerization agent of hydroxyapatite has been developed so as to enhance dispersibility of a polymerizable monomer in an aqueous medium. Such improvements are described in Japanese Examined Patent Publication No. 54-44313, Japanese Non-examined Patent Publication Nos. 5-222103, 6-220108, 7-102005 and 7-102006, in which a sedimentation half-life period and an electric conductivity are regarded as an index of improvement. In this index, as the sedimentation half-life period is long, and as the electric conductivity is low, it is suitable as a dispersant.
As uses of the water-difficult soluble phosphates for suspension polymerization, for example, in polymerization of polystyrene, there are known polymerization of general foamed polystyrene, polymerization of three-dimensional polymer fine particles as a low shrink agent for an unsaturated polyester resin composition (Japanese Non-examined Patent Publication No. 7-82305) and polymerization of a toner resin for electrostatic development. Further, production of fine polymer particles in use for a toner, there is known a method for dissolving a polymer in a solvent, dispersing the resulting solution in water to prepare a suspension, and removing the solvent from the suspension and it is reported that the water-difficult soluble phosphates for suspension polymerization are also effective in this method (Japanese Non-examined Patent Publication No. 9-230624).
In the suspension polymerization of styrene monomer for general purposes, there are known a dispersant type comprising organic surfactants such as polyvinyl alcohol and polyvinyl pyrrolidone and a dispersant type comprising a mixture of a water-difficult soluble inorganic salt such as calcium phosphate, hydroxyapatite and pyrophosphoric acid with an anionic surfactant such as sodium dodecylbenzene sulfonate, sodium xcex1-olefin sulfonate and lauric sulfuric ester sodium salt. However, when the organic surfactant is used as a dispersant, it is mixed into polymer particles to decrease heat stability, transparency or mechanical strength or to increase COD in a polymerization waste liquid, and thus it is not preferable even from the viewpoint of the waste liquid treatment.
On the other hand, in the dispersant comprising a mixture of a water-difficult soluble inorganic salt and an anionic surfactant, high suspension stability is obtained with a less amount of the water-difficult soluble inorganic salt, but when styrene monomer is suspension polymerized in the presence of this dispersant, the particle size distribution of the polymer particles obtained becomes broad.
Although polymer particles comprising polystyrene as the main ingredient obtained by suspension polymerization are generally extrusion-molded or injection-molded to produce various molded articles, when polymer particles having a broad particle size distribution are used, feeding of polymer particles into a molding machine varies to result in an increase in a defective ratio of the molded articles. Moreover, the three-dimensional polymer fine particles as a low shrink agent for unsaturated polyesters degrade uniform dispersibility, gloss, low shrink property and transparency of molded articles of the unsaturated polyesters. Furthermore, as to a toner for electrostatic development, recently, development by the use of a color toner has been demanded in place of a monochrome toner, and on the other hand, in compliance with a high density movement of information, a fine particle toner having a high resolution has been demanded. However, such demand can not be satisfied with a toner of a pulverized type and therefore a toner having a sharp particle size distribution between 0.5 and 10 xcexcm in particle size is demanded.
The uses of polymer particles containing polystyrene as the main ingredient include, for example, general uses of (1) to (3), use of (4) imparting uniform dispersibility, gloss, low shrink property and transparency to unsaturated polyester molded articles, and use of a toner (5), and particle sizes of the polymer particles required for such uses are determined as described below:
(1) Particle size ranging from about 0.4 to about 0.7 mm:
Cups for use in instant food
(2) Particle size ranging from about 0.7 to about 1.8 mm:
Various packages
(3) Particle size ranging from about 1.8 to about 3.0 mm:
Boards for building materials
(4) Particle size ranging from about 30 to about 80 xcexcm:
Low shrink agent for unsaturated polyesters
(5) Particle size ranging from about 0.5 to about 10 xcexcm:
Toner for electrostatic development.
As stated above, the particle sizes required are different according to the uses of the polymer particles. That is, if the particle size does not fall in the given particle size distribution, the intended properties can not be satisfied. When the particle size distribution of the polymer particles is broad, the yield lowers noticeably. As to an inorganic dispersant as well, one adaptable to the properties required of polymer particles is needed.
It is said that the stabilization effect of hydroxyapatite as an inorganic dispersant increases as the particle size of hydroxyapatite is fine for the reason that the surface active energy becomes large. However, hydroxyapatite particles in a slurry obtained from the manufacturing process are usually fine and agglomerate with ease, which causes the particle size of polymer particles to be ununiform. As is apparent from that, it is important that the hydroxyapatite particles are not only fine, but excellent in dispersibility. To attain this object, it is a general method to subject an apatite slurry to a shearing dispersion treatment and deagglomeration.
However, it is extremely difficult to disperse uniformly agglomerates of apatite fine crystals synthesized from a calcium hydroxide slurry and phosphoric acid, which is a general synthetic method.
An object of the present invention is to provide an inorganic dispersant of a calcium phosphate type compound which gives polymer particles having excellent quality as well as an intended particle size and a sharp particle size distribution.
Another object of the present invention is to provide a stabilizer for suspension polymerization comprising the above inorganic dispersant capable of producing polymer particles which have not only an intended particle size and a sharp particle size distribution, but are excellent in quality.
Yet another object of the present invention is to provide polymer particles using the inorganic dispersant, which have not only an intended particle size and a sharp particle size distribution, but are excellent in quality.
A further object of the present invention is to provide an unsaturated polyester resin composition containing the polymer particles, which is improved in low shrink property.
A still further object of the present invention is to provide a toner composition containing the polymer particles, which is improved in image density, fog, resolution, scattering and cleaning.
In light of the foregoing situation, the present inventors have made an extensive series of studies and have found out that a calcium phosphate type inorganic dispersant having a specific particle composition, particle shape, particle size and dispersibility, and specific surface area is capable of providing polymer particles which have not only a sharp particle size distribution with an intended particle size, but excellent quality.